Mp10

Embedded Control Applications II MP10-1

week lecture topics
10 Embedded - Servo-motor control
Control - Stepper motor control
Applications
II


Control of a DC servo-motor
- The control of a DC servo motor is another common
task in robotics / mechatronics; DC servo-motors
combine regular DC-motors with a gear-box and an
encoder/potentiometer to form a position control loop
- Being position controlled, the drive shaft of a servo-
motor can only assume a limited range of angular
positions (typically ±90° or less)
- The set-point is a Pulse Width Modulated (PWM)
signal with a period of commonly around 20 ms and
duty cycles of 2% to 10% (~0.5 ms to ~2.5 ms)


Encoder / Potentiometer

Power
amplifier

Gear-box DC motor


- Servo-motors have 3 wires: Vsupp, VGND and signal

- Only the signal line interfaces to the microcontroller –
the current carrying supply lines need to be connected to
a sufficiently powerful supply; typical supply voltages
range between 6 V and 30 V
red

10% duty cycle 6 V DC

black

yellow

20 ms


A small DC servo-motor is to be driven using an edge
aligned PWM signal on P7.3 (period: 20 ms). The duty
cycle is to be controlled by the analogue voltage applied
to ADC channel 4, which is also logged on a terminal
connected to ASC port S0 (57600 bps)
P5.4 5V
PORT 5 10 V
0V
PORT 3

PORT 7

S
RxD
P7.3
TxD 0V


- This is essentially the program developed in lecture
MP9; the period of the PWM signal has to be adjusted
to 20 ms and the duty cycle needs limited to the range
from 3 % (0.6 ms) to 10 % (2 ms)
- To produce the required 20 ms signal (50 Hz) the
PWM module needs to be set up with a pre-scale
factor of 1/64; a 12-bit resolution is to be expected
(mode: 0), i. e. 20⋅10-3/4096 ≈ 4.88⋅10-6 s ≈ 5 µs


- Running the modified program on the C167 confirms
a 20 ms period with pulses ranging from 0.6 ms
(setting: 0 %) to 2 ms (setting: 100 %)


- The terminal logs the current position as a percentage
of the range of admissible pulse widths


Control of a stepper motor [1]
- A stepper motor is an electromechanical device which
converts electrical pulses into discrete mechanical
movements

- The shaft or spindle of a stepper motor rotates in discrete
step increments when electrical command pulses are
applied to it in the proper sequence

- This sequence is directly related to the direction of
rotation of the motor shaft; the speed of the rotation is
directly related to the frequency of the applied pulse
sequence


Control of a stepper motor
Stepper motors have the following characteristics:
- The rotation angle of the motor is predictably related to the
input pulse pattern

- The motor has full torque at stand-still (if the windings are
energized)

- Precise positioning and repeatability of movement; good
stepper motors have an accuracy of 3 – 5 % of a step –
this error is non-cumulative from step to step

- Excellent response to starting, stopping, reversing


Stepper motors have the following characteristics:
- Stepper motors are brushless and thus very reliable; their
life span usually only depends on their bearings
- They allow for accurate open-loop control; the position
can be tracked simply by counting pulses
- They allow for very low speed synchronous operation
with loads that are directly coupled to the shaft
- Improper control may cause resonance phenomena
- Difficult to operate at extremely high speeds


Three different kinds of stepper motors exist:
- Variable-reluctance (VR) stepper motors consist of a soft
iron multi-toothed rotor and a wound stator

- Energizing the stator windings
with DC currents causes the
poles to be magnetized

- Rotation occurs when the rotor
teeth are attracted to the
energized stator poles


- Permanent-Magnet (PM) stepper motors (‘tin can’) are
low cost and low resolution type motors – typical step
angles range from 7.5° to 15°
- The rotor no longer has teeth (cf. VR
motor), but is magnetized with
alternating north and south poles
- The increased magnetic flux
intensity gives the PM motor an
improved torque characteristic


- Hybrid (HB) stepper motors combine the best features of
PM and VR type stepper motors; step angles vary from
3.6° to 0.9° (100 – 400 steps per revolution)
- The rotor is teethed with an
axially magnetized concentric
magnet around the shaft
- The teeth on the rotor help guiding
the magnetic flux; this leads to
increased performance


- The stator windings need to be energized in such a way
as to generate a rotating magnetic field; the rotor
follows this field due to magnetic attraction
- Two-phase example:
Energizing the windings
using a B-A-B-A-B-…
pattern leads to clockwise
rotation
- The rotational speed
depends on the frequency of
the alternating sequence


- The torque of a stepper motor depends on the step rate
as well as the intensity of the magnetic flux in the
windings which, in turn, is proportional to the drive
current
- A stepper motor usually has 2 phases; more complicated
designs with 3 and even 5 phases exist
- A pole can be defined as one of the regions where the
magnetic flux density is concentrated; there are poles
on both the rotor as well as on the stator
- Increasing the number of poles on rotor and/or stator leads
to smaller basic stepping angles (full step)


- Example: Unipolar 2-phase stepper motor with one pair
of poles per phase and one pair of rotor poles

- The flux can be
reversed by switching
the supply from phase
A/B to phase A/B


- Example: Bipolar 2-phase stepper motor with one pair of
poles per phase and one pair of rotor poles

- The flux can be
reversed by swapping
the + and - terminals of
the supply

- 8 full step positions are
possible (basic step
angle: 45°)


- The most common stepping modes are wave drive, full
step drive and half step drive
- In a wave drive system only one phase is energized at
any given time; sequence: A → B → A → B … leads
to steps from 8 → 2 → 4 → 6 (see MP10-18)
- In a full step drive system two phases are energized at
any time; sequence: AB → AB → AB → AB … leads
to steps from 1 → 3 → 5 → 7 (see MP10-18)
- A half step drive system combines the above two modes;
sequence: AB → B → AB → A → AB → B → AB → A
… (1 → 2 → 3 → 4 → 5 → 6 → 7 → 8)


- The advantage of full step drive over wave drive is
that, at any given time, a full step system uses 50% of
the available windings whereas the equivalent wave
drive system only uses 25%
- Furthermore, unipolar stepper motors only use 50% of
each winding to build up the magnetic flux; bi-polar
stepper motors on the other hand use the full winding
and therefore produce more torque
- Microstepping systems continuously vary the current
amplitude in the windings to break up a basic step into
many smaller discrete steps


- The stiffness of a stepper motor can be increased by
increasing its holding torque (TH); moving the drive shaft
away from an equilibrium position (rotor and stator poles
are aligned) leads to an opposing torque which increases
until TH is reached

- Beyond the holding
torque, the rotor position
becomes unstable and it
moves until it is aligned
with the next stator pole


- The torque vs. speed characteristic of a stepper motor
indicates its pull-in curve (defines a region at which
the motor can be started/stopped without loss of
synchronism)…

- … as well as its pull-out
curve (limits the slew
region, i. e. the region
within which the motor
can be operated without
loss of synchronism)



- The time-domain response of
a single step is subject to load
conditions and the maximum
required acceleration

- Driving the motor at frequencies near the natural
frequency of the rotor can lead to resonance; this
resonance manifests itself in a sudden loss or drop in
torque at certain speeds which can lead to loss of
synchronism


- The driver of a stepper motor can be implemented
using a microcontroller; the controller needs to
produce the required pulse sequence and interface to
an array of inverters or power MOSFETs
- This would only be done for educational purposes; in
the ‘real world’ a stepper motor driver chip would be
used (cost: ‘a few dollars’)
- This reduces the task to the provision of a pulse
sequence, the frequency of which defines the rotational
speed, and a directional signal (fw. / rev.)


- A typical design of a stepper motor driver is shown
below; note that the transistors of the power amplifier
often have to be implemented externally


Further reading:

[1]
Douglas W. Jones, Control of Stepping Motors – A
Tutorial, http://www.cs.uiowa.edu/~jones/step/,
accessed: January 2005
[2]
ELF/DWARF, Free Standards Group – Reference
Specifications, www.linuxbase.org/spec/refspecs/,
accessed: January 2005
[3]
The GCC Project, Free Software Foundation,
gcc.gnu.org/, accessed: January 2005

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